The present invention relates to a pneumatically operable screw driver preferably used for fixing a threaded fastening member to a wood material or the like.
Various pneumatically operable screw drivers have been conventionally proposed. According to a typical arrangement of the pneumatically operable screw drivers, a piston pushes a driver bit while an air motor rotates this driver bit. Thus, a threaded fastening member driven by the driver bit can rotate and protrude forward. For example, the Japanese Patent Application Kokai No. Hei 1-45579 discloses a screw driver which has a driver bit moving downward together with an air motor. The Japanese Patent Application Kokai No. Hei 5-261676, corresponding to the U.S. Pat. No 5,231,902 (DEP 4219032), discloses another screw driver which has a driver bit moving downward and a stationary air motor not moving downward together with the driver bit.
In general, the screw drivers take a relatively long time for driving and fixing a threaded fastening member due to their unique driving mechanism causing the driver bit to rotate and protrude simultaneously. An average time required in the practical screw fastening operations is approximately 0.1 to 0.2 sec which is more than 20 times the corresponding fastening time (approximately 0.005 sec) of the nail (i.e., non-threaded fastener) drivers.
Accordingly, in the actual operations of the screw drivers, operators are usually forced to hold a push lever linked to a manual operating valve of the screw driver for a while until the screw fastening operation is completely finished. The holding time, i.e., a waiting time, inherent to the screw drivers may be too much long for the operators who frequently use the nail drivers or other speedily performable pneumatic tools.
When the operator releases the finger force acting on the push lever at an early timing before finishing the screw fastening operation, the manual operating valve returns to the close position to stop the compressed air. The air motor and the piston cannot drive the driver bit any more at this moment. Thus, the driving operation of the screw is interrupted incompletely.